[to-be-committed] [RISC-V] Use pack to handle repeating constants

This patch utilizes zbkb to improve the code we generate for 64bit constants
when the high half is a duplicate of the low half.

Basically we generate the low half and use a pack instruction with that same
register repeated.  ie

pack dest,src,src

That gives us a maximum sequence of 3 instructions and sometimes it will be
just 2 instructions (say if the low 32bits can be constructed with a single
addi or lui).

As with shadd, I'm abusing an RTL opcode.  This time it's CONCAT.  It's
reasonably close to what we're doing.  Obviously it's just how we identify the
desire to generate a pack in the array of opcodes.  We don't actually emit a
CONCAT.

Note that we don't care about the potential sign extension from bit 31. pack
will only look at bits 0..31 of each input (for rv64).  So we go ahead and sign
extend before synthesizing the low part as that allows us to handle more cases
trivially.

I had my testsuite generator chew on random cases of a repeating constant
without any surprises.  I don't see much point in including all those in the
testcase (after all there's 2**32 of them).  I've got a set of 10 I'm
including.  Nothing particularly interesting in them.

An enterprising developer that needs this improved without zbkb could probably
do so with a bit of work.  First increase the cost by 1 unit. Second avoid
cases where bit 31 is set and restrict it to cases when we can still create
pseudos.   On the codegen side, when encountering the CONCAT, generate the
appropriate shift of "X" into a temporary register, then IOR the temporary with
"X" into the new destination.

Anyway, I've tested this in my tester (though it doesn't turn on zbkb, yet).
I'll let the CI system chew on it overnight, but like mine, I don't think it
lights up zbkb.  So it's unlikely to spit out anything interesting.

gcc/
	* config/riscv/crypto.md (riscv_xpack_<X:mode>_<HX:mode>_2): Remove '*'
	allow it to be used via the gen_* interface.
	* config/riscv/riscv.cc (riscv_build_integer): Identify when Zbkb
	can be used to profitably synthesize repeating constants.
	(riscv_move_integer): Codegen changes to generate those Zbkb sequences.

gcc/testsuite/

	* gcc.target/riscv/synthesis-9.c: New test.

0 files changed, 0 insertions, 0 deletions